The present invention relates to thermoplastic polyesters which are melt processable, and more particularly to thermoplastic polyesters containing isomers of biphenyldicarboxylic acid.
Thermoplastic polyesters containing isomers of biphenyldicarboxylic acid are known to be useful in the fabrication of articles such as fibers which are useful as tire cords, reinforcement in hoses, cables, conveyor belts or composite structures with matrixes prepared from other resinous materials. These polyesters are also known to be useful in the manufacture of films which have excellent solvent and chemical resistance, low flammability and good electrical insulating properties.
Homopolyesters of 4,4'-biphenyldicarboxylic acid and certain aliphatic diols and copolyesters of this diacid and mixtures of aliphatic diols are disclosed in the literature. For example, Ezard, in the Journal of Polymer Science, Volume 9(1), page 35, (1952), discloses the homopolyester of 4,4'-biphenyldicarboxylic acid and ethylene glycol; and Meurisse et al., in the British Polymer Journal, Volume 13, page 57 (1981), disclose the homopolyesters of 4,4'-biphenyldicarboxylic acid and 1,4-butanediol and 1,6-hexanediol. Copolyesters of 4,4'-biphenyldicarboxylic acid and mixtures of aliphatic diols are disclosed in U.S. Pat. No. 2,976,266.
The homopolyester of ethylene glycol and 4,4'-biphenyldicarboxylic acid melts at 343.degree. C., too high to be molded from its melt. This polyester and other similar polyesters containing units derived from aliphatic diols begin to degrade at significant rates at temperatures significantly above 300.degree. C. To make melt processing possible, it is necessary to change the polymer's composition slightly to lower its T.sub.m into a range where it is more thermally stable. A process for doing this comprises incorporating terephthalate units into poly(ethylene 4,4'-biphenyldicarboxylate) as disclosed by Krigbaum et al. in the Journal of Polymer Science, Polym. Letters, Volume 20, page 109 (1982). However, copolyesters of 4,4'-biphenyldicarboxylic acid, terephthalic acid and ethylene glycol tend to be brittle and have only limited utility.
It would be highly desirable to provide thermoplastic polyesters which are melt processable, which are not brittle and which do not have the disadvantages exhibited by the prior art polymers.